An X-ray CT (Computed Tomography) apparatus including an area detector reconstructs volume data based on the projection data obtained by scanning an object along a circular orbit. Projection data corresponds to the data obtained by performing line integration of X-ray attenuation coefficients along X-rays. In this case, a region (to be referred to as a measurement data complete region hereinafter) where projection data in an angular range (e.g., 360°) necessary for reconstruction processing is completely prepared in the form of measurement data is a region (a region inside the shape of a truncated bicone) inside the rotating body obtained by, for example, rotating the lower base of an isosceles trapezoid about a rotation axis. A mask which is covered to inhibit the execution of reproduction processing is provided for a region (to be referred to as a mask region hereinafter), of a region where projection data has been acquired, where projection data in an angular range (e.g., 360°) necessary for reconstruction processing is not completely prepared in the form of measurement data. FIG. 18 is a view showing a measurement data complete region and mask regions together with an X-ray tube and an X-ray radiation range. In order to reconstruct the volume data obtained by approximating the rotating body to an almost cylindrical shape by reducing mask regions (this processing is called mask region reconstruction or body axis direction region expansion reconstruction and will be called mask region reconstruction hereinafter), it is necessary to perform dedicated reconstruction using measurement data and interpolation data generated by interpolating the measurement data.
Step and shoot scanning uses a plurality of volume data generated by using mask region reconstruction when coupling the data along the body axis direction of the object. The volume data coupled along the body axis direction of an object will be referred to as a wide-volume. The use of mask region reconstruction can reduce a region (to be referred to as an overlap region hereinafter) where overlapping occurs between a plurality of volume data to be coupled. A reduction in overlap region will contribute to a reduction in exposure to radiation of an object in step and shoot scanning and generation of wide-volume.
It is not, however, that it is possible to execute mask region reconstruction under any conditions. For example, a portion exhibiting a large change in CT value in the body axis direction is located at a cone angle end, the quality of an image at the portion deteriorates. For this reason, in mask region reconstruction, image quality deteriorates depending on the portion.